The cartridge is made of glass and has a dental anesthetic solution filled therein for local anesthesia. There are cartridges of 1.8 ml and 1.0 ml of anesthetic in volume and they are different in length due to the different volumes of anesthetic and due to the fact that the cartridges have identical inner and outer diameters.
In either case, a motorized syringe may used for administration of anesthesia by loading a cartridge into a cartridge holder, detachably connecting the cartridge holder to a coupling of the motorized syringe, and affixing a double pointed needle to the tip of the cartridge. The motorized syringe comprises a pusher rod for pushing a plunger rubber plug in the cartridge to deliver anesthetic solution from the cartridge through the needle, and an operating mechanism for advancing the plunger rod. The operating mechanism generally, includes a pinion rotated through a power transmission from a drive unit which includes a drive motor and a reduction gear, and a rack formed on the pusher rod and in engagement with the pinion. In order to deliver the entire anesthetic solution from the cartridge loaded in the cartridge holder, the pusher rod must be moved a distance equal to the length of accommodation of anesthetic solution in the cartridge while pushing the plunger rubber plug in the cartridge by advancement of the pusher rod with its rack engaging the rotating pinion. The distance of movement of the pusher rod is defined by bringing a limit switch into an off-state by the rear end of the pusher rod to stop the rotation of the motor. After delivery of anesthetic solution, the cartridge holder is detached from the coupling and the power transmission from the drive unit is interrupted to bring the pinion into an idle state. The pusher rod can thus pushed into the syringe body the identical distances by hand. In this process, the limit switch is brought into an on-state by the outer surface of the pusher rod. The pusher rod must be configured to extend rearwardly from the pinion by a length that is equal to the length of accommodation of anesthetic solution in the cartridge plus a length equal to a distance between the pinion and the limit switch is. To this end, the pusher rod for the motorized syringe must be long in size, and consequently, the motorized syringe must be designed in a long and bulky configuration. From the foregoing, an appearance of a relatively compact cartridge-type motorized syringe has been strongly desired from dental practitioners.